The present invention relates generally to class AB amplifiers, and more particularly, to a monolithic class AB low noise amplifier having feedback and self bias. The preferred application of such a circuit is for (monolithic low cost high dynamic range, low noise CMOS) FR receivers used in FM stereo systems, although performance can extend to higher frequency ranges.
Conventional open-loop class AB amplifier architectures have a number of limitations, as is illustrated in a typical prior art design shown in FIG. 1. Such limitations include a complicated biasing scheme that requires active devices (transistors M.sub.B, M.sub.2, M.sub.3, and a bias network IREF (not shown)) to bias-up input gate voltages of NMOS (M.sub.1) and PMOS (M.sub.2) devices. A common mode amplifier (transistors M.sub.4 -M.sub.7) is also needed to bias the DC operating point at the output. The common mode amplifiers are required to set up a DC voltage of V.sub.B1 at V.sub.OUT1 and V.sub.B2 at V.sub.OUT2. The NMOS and PMOS (M.sub.1, M.sub.2) devices do not receive adequate gate-to-source voltage (V.sub.gs) for the desired current for a given aspect ratio (W/L) and results in a lower transconductance (gm).
The open-loop class AB design of in the prior art class AB amplifier architecture of FIG. 1 achieves high gain using a two-stage cascade approach. Problems associated with the cascading design involves an interstage that does not match impedance (resulting from directly coupled stages), making the gain and seminal impedances of the amplifier highly process dependent. Also, prior art amplifiers require multiple stages to achieve high gain. Another limitation is that an AC decoupling network (C.sub.1 -C.sub.4) only allows the amplifier to work at high frequencies in a narrow bandwidth. It is therefore difficult for the amplifier to work at FM frequencies due to the size of the capacitor in the AC decoupling network. An external matching network (M.sub.B, M.sub.L) is also required to match the respective input and output ports of the low noise amplifier to 50 ohms, and due to the lack of feedback, are also only narrowband matches. The feedback structure of the present invention allows matching over a wide bandwidth.
Accordingly, it is an objective of the present invention to provide for an improved monolithic class AB low noise amplifier having feedback and self bias that allows impedance matching over a wide bandwidth. It is also an objective of the present invention to provide for an improved monolithic class AB low noise amplifier wherein feedback allows a reduction in sensitivity to process variations for the desired impedance match.